Onch Slugfest

Onch slugs (family Onchidiidae) are a group of shell-less gastropod molluscs in the same class as the more familiar snails and slugs. However, they are mostly found in coastal habitats or in the inter-tidal area.

Like snails, onch slugs have a pair of simple eyes on stalks that are only good for detecting light. Most of them look like shell-less snails or a bumpy brownish lump - the word "onch" literally means lump. They are not the prettiest of organisms, but their humble appearance grants them excellent camouflage. It definitely follows the wise adage that if you do not want to be eaten, do not look like food!

One unique characteristic of onchs is that they breath air using a simple lung, unlike marine slugs such as nudibranchs that get oxygen via feathery gills. Hence, onchs are considered pulmonates (from the Latin word pulmonarius, meaning of the lung). In this aspect, onchs are more similar to land snails and slugs.

Since onchs need to breath air, they are usually found above the water level on rocks, tree trunks or even man-made surfaces and in the inter-tidal area during low tide where they feed on algae and organic detritus. They scrape bits of algae off the surface with a rough rasp-like mouthpart called a radula.

Coupling in onchs is a strange affair. These slugs are hermaphrodites bearing both male and female organs. When they get together, they jab each other with hard and sharp love darts in a courtship ritual prior to mating where sperm is exchanged.

A soft-bodied animal living out of water will often face 2 major problems that is dangerous to its health - drying out and predators. Onch slugs are extremely well suited to their habitats and combat desiccation and predators with both structural and behavioural adaptations.

A thick mucus is secreted, covering the skin of the animal to prevent desiccation. In addition, the mucus is also thought to be foul tasting, and any predator attempting to swallow an onch will get a mouthful of a yucky gooey lump. In some species, the mucus leaves a trail for the onch to follow back to their own home.

Some onch can also burrow into the substrate to avoid heat, strong currents and predators. This one below burrowed into the mud within seconds upon sensing danger. Couple excellent camouflage, multi-purpose mucus and a nifty burrowing behaviour, onchs have a lot going for them where survival is concerned.

Unfortunately, on the whole, these charming creatures remain a poorly studied group and little is known about the biology of onch slugs in Singapore. Hopefully, more work can be done in this area to demystify these amazing little slugs.

Further reading:

1. McFaruume, I. D. 1980. Trail-following and trail-searching behavior in homing of the intertidal gastropod mollusc, Onchidium verruculatum. Marine and Freshwater Behaviour and Physiology 7(1): 95-108.
2. Ng, P. K. L. and Sivasothi. N. 1991. Mangrove slugs (Onchidiidae). A Guide to the Mangroves of Singapore Volume 2. Singapore Science Centre.

Estuarine Crocodile: An Ambush Predator

Last month, Peiting of The Simplicities in Life wrote about an estuarine crocodile (Crocodylus porosus) stalking a common sandpiper (Actitis hypoleucos). Since the crocodile did not end up with the bird in its jaws, it led to some speculation that it could have been just on its way up to its favourite busking spot.


Today at the mangrove reserve, two estuarine crocodiles were present and a similar stalking event occurred. One of the subadult crocodiles glided stealthily from the west bank of Sungei Buloh Bersah and approached a common sandpiper on the opposite bank.

Here, it can be seen how having its eyes and nostrils positioned on top of the head allows the crocodile to see above the water and breathe while keeping the rest of its body hidden. Since the species is able to hold its breath comfortably for up to 5 minutes and if forced, stay submerged for more than an hour, it is likely that this exercise which happened within 5 minutes could have been solely to watch the sandpiper.

The reptile then swam past where the sandpiper stood and positioned itself where the bird was heading - in other words, was leading the bird. In an ambush set up, what the crocodile is doing is essentially predicting where its prey will be and positioning itself in the predicted path of its prey.

It seemed that the crocodile's prediction was right and the sandpiper passed right in front of the crocodile. A few moments passed. Both animals appeared to hesitate and that gave the sandpiper an opening to take off, landing on the fallen tree behind.

Despite the fact that no bird ended up in the jaws of this crocodile, this event strengthens the view that the previous record was probably not born out of coincidence and sheds some light on the hunting technique employed by one of our top mangrove predators. As for the crocodile, after the sandpiper's timely flight, it turned itself around to face the opposite end of the river bank.

Though a small bird like the sandpiper may seem like an insignificant meal for a reptile about 1.6 m long, for a "cold-blooded" animal which does not require to burn energy to maintain a constant body temperature, a small bird meal can probably go a long way.

Also, bird-eating is in agreement with studies done on the diet of the species where smaller crocodiles fed mainly on insects, crabs and shrimp then graduate to larger vertebrate prey such as fish, birds and mammals.

After all, it would be a waste for an animal to have evolved one of the strongest bites over the millions of years of its existence if it was hunting only shrimps, beetles and halfbeaks.

Further reading:

1. Taylor, J. A. 1979. The foods and feeding habits of subadult Crocodylus porosus Schneider in Northern Australia. Australian Wildlife Research 6(3): 347-359.

Mangrove Plants 1: Getting to the root of things

Often, I get besieged by botanists (including closet botanists and ornithologists) due to my botanical inaptitude. So here I am atoning for my zoological bias and aversion to all things from kingdom Plantae.

To start off, the swampy mangrove is a good place to introduce and highlight how highly adapted plants can be since there are less than 30 true mangrove plant species in Singapore and many have developed unique adaptations for the habitat.

Here, conditions on the ground is mostly muddy and unstable with low oxygen and high salt content from the sea. This poses various challenges to plants - imagine living in salty quick sand!

Hence, from ground up, plants that have evolved roots for support and coping with low oxygen and salt immediately gain a competitive advantage over those that do not.

One such group is Rhizophora. If asked how does one increase stability? Rhizophora's answer would be to grow more legs.

Members in this group develop prop or stilt roots that branch and loop from the trunk and branches. The result is a network of charmingly grotesque gothic pillars at the bottom of the tree.

When exposed during low tide, these long roots also help with oxygen intake.

These roots also have an additional trick that blocks salt from seawater entering the roots in a process called ultrafiltration.

In Bruguiera and Ceriops, the trees send their roots out far and wide, and increases anchorage by having bent, kneed roots at intervals that resemble the legs of people doing sit-ups on the ground.

This curious structures not only adds stability by increasing surface area of attachment and binding more sediment, but the parts that stick out also helps obtain oxygen.

In Bruguiera, the roots are also said to be able to perform ultrafiltration to remove salt.

Avicennia and Sonneratia also spread their roots far and wide for stability, but have spike-like breathing roots in place of kneed roots.

In spy or adventure movies, characters sometimes hide underwater and breathe using straws or tubes poking out of the surface. Breathing roots, or pneumatophores, work in a similar way.

The roots of Sonneratia are capable of ultrafiltration, but not in Avicennia. The latter, has other tactics which will be discussed in a future post.

But what is most interesting is how unrelated (or only distantly related) groups have somehow acquired similar adaptations at root level for mangrove living. Rhizophora, Bruguiera and Sonneratia can exclude salt when taking in seawater (ultrafiltration), while Avicennia and Sonneratia solve oxygen shortage with spike-like breathing roots.

This phenomenon is what scientists term as convergent evolution. Another example would be how dolphins (order Cetacea) and manatees (order Sirenia) have separately evolved fluked tails for swimming. Oops, mentioned mammals in a plant post.

Further reading:

1. Ng, P. K. L. and Sivasothi. N. 1991. How plants cope in the mangroves. A Guide to the Mangroves of Singapore Volume 1. Singapore Science Centre.